1. Field of the Invention
The present invention concerns the removal of oxides of nitrogen (herein referred to as "NO.sub.x ") from flue gases and, more particularly, the removal of NO.sub.x from a continuous stream of flue gas by reacting said flue gas in a bed, a layer or a structure of adsorption means having catalytic effects with ammonia (herein referred to as "NH.sub.3 ") as a reducing agent.
2. Prior Art
West German patent application No. 2 911 712 divulges a two-stage method of pollutant removal providing for a first-stage moving bed for the removal of a substantial part of oxides of sulfur contained in flue gas and a second-stage moving bed containing a granular carbon-containing adsorption agent for the catalytic reduction of NO.sub.x to molecular nitrogen following the controlled addition of gaseous NH.sub.3 and for the removal of further oxides of sulfur by a method of the nature mentioned hereinabove. However, if activated carbon is used as a catalyst for such a method of NO.sub.x control using NH.sub.3 as a reducing agent, large quantities of NH.sub.3 must always be present on and retained by the surfaces of said activated carbon, thereby necessitating a high NH.sub.3 partial pressure implying a high NH.sub.3 concentration in said flue gas exceeding the stoichiometric concentration necessary for the conversion of the NO.sub.x contained in said flue gas. As soon as the NH.sub.3 concentration exceeds the activated carbon equilibrium load, substantial quantities of NH.sub.3 will therefore pass through said activated carbon and will be contained in the flue gas downstream of said activated carbon bed.
All known methods of the nature described provide for a continuous addition of the reducing NH.sub.3 to such flue gas upstream of said activated carbon bed at a rate depending upon the desired NO.sub.x removal efficiency. If conventional methods are used, a high NH.sub.3 content of the flue gas downstream of said activated carbon bed is inevitable, if a high NO.sub.x removal efficiency such as the removal of more than 70% of the NO.sub.x originally contained in such flue gas is desired.
According to the state of the art hitherto known, if both oxides of sulfur and NO.sub.x are removed from flue gas, excess ammonia is retained in a separate unit through which activated carbon from the sulfur dioxide removal unit is continuously cycled. Said activated carbon is loaded with acidic compounds from said sulfur dioxide removal unit and thence substantially decreases the NH.sub.3 content of said flue gas. However, if activated carbon is exclusively used for NO.sub.x control, the quantity of activated carbon loaded with acidic compounds is insufficient for effective NH.sub.3 control.